Ah, herein lies the problem:
"all paths in /etc/mtab have to be canonicalized absolute paths"
Because we're using lvm2 and device mapper, the canonicalized absolute
path to /dev/TAFT_CLUSTER/TAFT_CLUSTER0 is actually
/dev/mapper/TAFT_CLUSTER-TAFT_CLUSTER0. I lost access to this cluster,
but I recreated it on another. After I modified /etc/mtab to have the
full canonicalized absolute path, mount -a found it as well.
In version 5.1, the mount helper puts the resolved path into mtab,
so we don't have the problem there. I'll backtrack it from here.

I haven't backtracked this yet, but the fact remains: the problem does
not occur in rhel5.1. Seeing as how it's too late to get anything into
5.0.z anyway, rather than waste any more of my time trying to dig through
code, I'd like to just close this bug as CURRENTRELEASE. Is that alright?